How does sharing a file affect quotas in Ubuntu one?

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If person A is at 4Gb of their 5Gb quota, and person B shares a folder, F, with them with 2Gb data in it, has person A just gone 1Gb over quota? Or does all of the content of F always belong to B's quota?
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I have VirtualMin installed on a development server running CentOS 6.5. None of the users have explicit quotas, yet periodically I get write issues when running as that user. If I run as root, I have no problems writing.